The goal of this proposal is to elucidate the mechanism of Ca2+ sensing in a C-cell line that is responsive to physiologic changes in extracellular Ca2+, rMTC 44-2. This cell line serves as a good model of Ca2+ sensing, particularly applicable to endocrine tissues of the Ca2+ homeostatic system. The specificity of the Ca2+-sensing apparatus will be examined using polyvalent cations other than Ca2+. The intracellular messengers involved in the transduction of the external Ca2+ signal will be examined. Single cell spectrofluorimetry will be used to study the cytosolic Ca2+ responses to stimulation. Polyphosphoinositide and cyclic nucleotide systems will be examined, and G-protein involvement will also be evaluated. The Endocrinology Division at the Brigham and Women's Hospital consists of a unique combination of faculty, comprising both clinical and basic science investigators, and physical resources. Investigators here have a broad spectrum of expertise, including electrophysiology of endocrine tissues, single cell spectrofluorimetry, receptor physiology, molecular biology and cell biochemistry. Clinical investigators include experts in Ca2+ homeostasis and osteoporosis, hypertension, neuroendocrinology and circadian biology. This diverse group meets for several weekly research and clinical conferences. The members of this department have at their disposal several fully equipped modern laboratories and an administrative core. The resources of the Harvard Medical School and Brigham and Women Hospital are available, including the opportunity to participate in graduate school courses and consult with members of other departments.